Conventional single layer photoresists are unable to perform satisfactorily in the resolution of features smaller than about 1 .mu.m. Typically, a resist is coated at a nominal thickness of 1 to 1.5 .mu.m over a support, which can be a semiconductor substrate (also known as a wafer), having variable topography and reflectivity. As a consequence of such nonuniformity, the combined effect of exposure and development varies from point to point on the wafer. For example, a thin region overlying a highly reflective aluminum support will be overexposed and overdeveloped relative to a thick region overlying a less reflective support. As a result, an intolerable degreee of line and space dimension variation will be evident across the wafer.
Trilevel resists have been utilized in the formation of electronic devices and are especially suitable for resolving features smaller than 1 .mu.m. Such resists can comprise an intermediary layer formed on an underlying planarizing layer, which is generally deposited on the wafer. An overlying third layer, delineable by exposure to radiation and by subsequent tratement with a suitable developing medium, is formed on the intermediary layer. The desired pattern is transferred to the intermediary layer through conventional techniques such as dry or wet etching, causing an uncovering of the underlying layer. The uncovered regions are then removed with an oxygen plasma etch. The intermediary layer, formed from materials such as silicon dioxide, is unaffected by the oxygen plasma, and thus avoids degradation of the transferred pattern. Trilevel resist systems, however, involve many discrete processing steps.
In an attempt to reduce processing steps and associated costs, there has recently been a significant effort to develop a bilevel system yielding the advantages, i.e. planarization and high resolution, of a trilevel system. Attempts have been made to combine the attributes of the intermediary layer and the overlying layer into a single layer which functions as an imaging layer and as an etch mask. In such a case, the layers must be designed to allow the planarizing layer to etch at a rate much faster than the resist layer. Thus, the resist must erode slowly and protect the underlyign planarizing layer as the uncovered planarizing layer is removed. Such attempts have been summarized by Reichmanis et al. in "Approaches to Resists for Two-Level RIE Pattern Transfer Applications," Solid State Technology, August 1985, p. 130-135. Many of these attempts involve resists comprising silicon-containing polymers. For example, U.S. Pat. No. 4,481,049 describes a polymer derived from a silicon-containing methyl methacrylate monomer copolymerized with sensitizing oximer or indanone monomers. None of these attempts, however, as yet, are entirely satisfactory. For example, it is desirable to maintain the weight percentage of silicon in such polymers at more than about 6 percent to obtain adequate resistance to the oxygen plasma etch. At such levels of silicon, the prior art polymers exhibit inadequate dimensional stability; i.e. they tend to flow or soften during processing at temperatures greater than 125.degree.C., resulting in image distortion. Moreover, such prior art silicon-containing polymers have poor dissolution rates. Resist dissolution rate is important because it is realted to development time and can affect critical dimension control. Development time is the amount of time necessary for the developer to contact the resist composition to achieve image clean-out. Submicron dimensions of undeveloped composition tend not to retain their tolerances if lengthy development times, for example, greater than about 3 minutes, and/or high base concentration developers are required due to poor dissolution rate. Furthermore, many of the prior art silicon-containing resists are not aqueous-developable. There has been a need, therefore, prior to this invention, to find improved resist compositions which have superior thermal stability at temperatures greater than 125.degree.C., which exhibit excellent dissolution rates and can be developed in an aqueous developer, and which are capable of achieving submicron resolution in a bilevel mode.